scope: full-screen figures, second windows, gated measurements, per-configuration events

A batch of view and instrument work, with the probes and docs for each.

- **Full screen per figure row**: a toggle on each row's top-right corner, revealed by
  pointer proximity (`kMaximizeRevealPx` around its placed rect, per row, read from an
  overlay-level motion controller so a revealed button under the pointer cannot flicker);
  the plot menu offers the same as *Full screen this figure* / *Restore the layout*. The
  arrangement is saved and restored through `RowState`, so nothing plotted moves. A layout
  of one figure carries no toggle.
- **Second window**: `open_scope_window()` copies a figure (cells, width shares, signals in
  their colours) or opens an empty one, with its own `RowSet` and `PlotArea`; the main
  window's tick binds, acquires and repaints it, mirroring the time window and pin. Entries:
  the plot menu's *Open this plot in a new window* and the overflow menu's *New scope window*.
- **Layout editor from the plot**: the plot menu gains *Edit the layout...* / *Done editing
  the layout*, flipping the app's one toggle.
- **Measurements gate**: `MeasureOptions::gate` restricts a measurement to the held/pause
  snapshot (live falls back to the live ring), persisted with the session and shown in the
  measure dialog.
- **Crosshair axis badges**: a preference (`crosshair_axis_labels`, on by default) and the
  plotter's badges where the lines meet the axes, in the axes' own formats; the plot menu
  and the probe read them.
- **Event cache per configuration**: `EventStore` is keyed `<source>/<configuration>/`,
  `retarget_event_store()` follows every configuration switch, and a pre-configuration cache
  (loose files) is adopted into the configuration's folder on first start.
- **GTK4 lifecycle**: the main window's handlers move from the non-existent `"destroy"`
  signal to `close-request`; scope windows are closed with it and the application quits
  explicitly (the exit run's hang). The exit probe closes with `gtk_window_close()`.
- Probes: `[maximize-probe]` (single-figure rule and the corner reveal), `[scope-probe]`,
  `[split-probe]` cell crosshair leg, `[prefs-probe]` store keying, `[configs]` store key,
  `[rowmenu]` layout-editor entry; docs, help pages, README and AGENT.md updated.

Also fixes a probe-only use-after-free: `toggle_maximize_figure_row` replaces the figure
object, so the area must be re-pointed (the button's path does; the probe's raw call did not).

docs/claude/invariants-session-and-export.md

@@ -192,3 +192,15 @@ of.
 
 Nothing is lost by switching: the dialog saves the current session *before* loading the picked
 one, which is the only ordering in which a switch cannot lose what it switches from.
+
+- **The event cache is keyed on the source *and its active configuration*, and the key moves
+  with the switch.** An event's trigger, its channel set and its units are the configuration's,
+  so the directory is `<base>/<host>-<port>/<configuration>/` (`EventStore`'s third constructor
+  argument) and `retarget_event_store()` -- called next to every `App::config_name` write and
+  resolving the name from `active_configuration()` when it is empty -- re-derives it, refreshing
+  both views of the list only when the directory changed. A store built without a configuration
+  keeps the legacy `<base>/<source>/` path, and the **first construction under a configuration
+  adopts the loose event files it finds there** (`*.npy`/`*.txt`/`*.bmp`, moved into the
+  configuration's folder): an upgrade keeps the events it had. The key is a *sanitized* path
+  component, the same rule the configuration files use, so a name typed with a slash cannot
+  leave the directory.